As water heats in a pot on a stove, it might produce some mist above its surface right before the water begins to roll. What is happening?

When a liquid is heated, the molecules gain kinetic energy. As the liquid begins to boil they have enough energy to break the intermolecular attractions between their neighbors. This happens first at the surface before the volume below has enough energy to boil. Thus you see high energy water molecules escaping from the surface as mist.